1. Field of the Invention
The present invention relates to an automobile bumper structure.
2. Description of the Related Art
A bumper structure (skeleton structure), which serves as a reinforcing member, is disposed in a bumper fascia at the front end or the rear end of an automobile such as a passenger car or a truck. A bumper structure includes a bumper reinforcement, which has a beam-like shape, and a bumper stay, to which the bumper reinforcement is fixed. The bumper stay is fixed to a vehicle body at two positions in the vehicle-width direction. To date, methods using a bolt and a nut or welding have been used to join a bumper reinforcement to a bumper stay.
As reduction in the weight and increase in the variety of design of a vehicle body have progressed in recent years, increase in the strength and reduction in the size of a bumper structure have been required. A bumper structure in which a bumper reinforcement is joined to a bumper stay using a bolt and a nut or welding has a problem in that, for example, the bolt may become broken or the bolt or nut may bite into the bumper reinforcement and break the bumper reinforcement. In order to prevent this, the sizes of bolt and nut or the thickness of a weld may be increased. In this case, however, a problem arises in that the weight of the bumper structure increases.
Japanese Unexamined Patent Application Publication No. 2004-237818 describes a bumper structure that is made by electromagnetically expanding a pipe. With the technology described in Japanese Unexamined Patent Application Publication No. 2004-237818, as illustrated in FIG. 2 of the Publication, a shaft portion of a bumper stay is fitted into a hole in a bumper reinforcement, an electrode is inserted into the shaft portion of the bumper stay, and a large pulse current is applied to the inductor, thereby expanding the shaft portion of the bumper stay and forming a flange and a bulging portion. By using the technology described in Japanese Unexamined Patent Application Publication No. 2004-237818, the number of components can be reduced from that of an existing structure in which a bolt and a nut are used. Moreover, increase in man-hours needed to assemble the bumper structure can be prevented. Furthermore, a problem of thermal distortion does not occur because a large amount of heat due to welding is not generated.
Japanese Unexamined Patent Application Publication No. 2010-51974 describes a method for joining a pipe to a member-to-be-joined by expanding the pipe using a segment die. With the technology described in Japanese Unexamined Patent Application Publication No. 2010-51974, a pipe is inserted into an insertion hole in a member-to-be-joined and a portion of the pipe that is inserted into the insertion hole is pressed outward in the radial direction of the pipe by using die segments of a die that is disposed in a hollow space in the pipe, thereby expanding the inserted portion of the pipe and joining the pipe to the member-to-be-joined.
However, the technology described in Japanese Unexamined Patent Application Publication No. 2004-237818 has a problem in that the dimensions of a pipe for which this technology can be used are limited for the following reason. That is, as illustrated in FIG. 2 of the Publication, electromagnetic expansion is performed by disposing an inductor, which is a tool, in a pipe to be expanded, and it is necessary to dispose the inductor in a smaller space when a smaller pipe is used. Moreover, because a small inductor has low strength and durability, this technology has a problem in that it is not suitable for mass-production. Furthermore, in order to increase the strength of a bumper structure, a bumper stay may be formed from a pipe made of a high-strength material. To expand a pipe made of a high-strength material, it is necessary to use an inductor that generates a large expansion force. Because a space for disposing an inductor is limited, the technology has a problem in that it cannot be used to make a bumper stay from a pipe made of a high-strength material. That is, the technology described in Japanese Unexamined Patent Application Publication No. 2004-237818 has a problem in that it cannot be sufficiently adapted for increase in the strength and reduction in the size of a bumper structure.
The technology described in Japanese Unexamined Patent Application Publication No. 2010-51974 has a problem in that a sufficiently large expansion force cannot be obtained when the diameter of a pipe to be expanded is small because only a small tool can be inserted into such a pipe. Moreover, this technology, which uses a segmented die, has a problem in that a pipe cannot be uniformly expanded over the entire circumference. That is, the technology described in Japanese Unexamined Patent Application Publication No. 2010-51974 has a problem in that it cannot be sufficiently adapted for increase in the strength and reduction in the size of a bumper structure.